Not applicable.
Not applicable.
1. Technical Field
This invention relates to concrete and controlled low-strength materials having improved characteristics, reduced limestone content, and electrical conductivity such that when used in construction, the material is capable of conducting electrical charges such as those resulting from a lightning strike. Further, the concrete and controlled low-strength materials comprise a high carbon content fly ash, thus providing a means for utilization of a product usually considered a by-product, or a waste product, of coal burning power generation.
2. Background Information
It is widely known that fly ash can be incorporated into concrete as a pozzolanic constituent. For example, U.S. Pat. No. 4,268,316 teaches the preparation of a masonry cement comprising from about 10 to 25 percent fly ash, in combination with portland cement and kiln dust. The fly ash utilized in this patent comprises any of those fly ashes which meet the requirements of ASTM (American Society for Testing and Materials) C 618, xe2x80x9cStandard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete.xe2x80x9d The patent goes on to postulate that fly ash may be utilized as a mineral admixture in masonry cement in place of natural pozzolans, siliceous or siliceous and aluminous materials which chemically react with calcium hydroxide to form compositions possessing cementitious properties.
It is also known that fly ash can be incorporated into controlled low-strength materials (often referred to as xe2x80x9cCLSMxe2x80x9d). In the publication xe2x80x9cControlled Low-Strength Materialsxe2x80x9d, reported by American Concrete Institute Committee 229, June 1999, there is provided a description of controlled low-strength materials along with certain ingredient mixtures used to produce CLSM. Controlled low-strength materials are broadly defined in this publication as self-compacted, cementitious materials used primarily as a backfill in place of compacted fill. Conventional CLSM mixtures usually consist of water, portland cement, fly ash, and fine or coarse aggregates. Some CLSM mixtures consist of water, portland cement and fly ash. However, CLSM is not to be considered as a type of low-strength concrete. This publication also defines CLSM as a material that results in a compressive strength of 8.3 MPa (1200 psi) or less at the conventional 28 day testing period (typically without compaction), and notes that most current CLSM applications require unconfined compressive strengths of 2.1 MPa (300 psi) or less at the conventional 28 day testing period in order to allow future excavation. This publication makes reference to certain examples of CLSM mixtures which include fly ash. U.S. Pat. Nos. 5,951,751 and 4,374,672 also disclose the use of fly ashes which meet the requirements of ASTM C 618 in controlled low-strength materials. In U.S. Pat. No. 5,951,751, one composition includes 1-15% of a lime component, 10-45% of a pozzolanic material (which may be ASTM C 618 Fly Ash), 20-95% aggregate and 10-20% water by weight; and in U.S. Pat. No. 4,374,672, one composition includes 45-80% ASTM C 618 Fly Ash, 1-6% cement and 20-50% water by weight.
It is also known that fly ash is a voluminous by-product of coal burning electrical power generation plants, presenting a possible environmental disposal issue. While those fly ash varieties which meet the requirements of the ASTM Standard Specification C 618 for classes C and F are used as additives to concrete, those fly ash materials which have an excessively high carbon content may not be so used. Accordingly, much of this type of fly ash is relegated to land fill, a less than desirable solution from an environmental viewpoint.
Air dried concrete is considered a reasonably good electrical insulator, having a resistivity on the order of 106 ohm-cm, with oven dried concrete having a resistivity on the order of 1011 ohm-cm. Moist concrete, on the other hand is an electrolyte having a resistivity on the order of 104 ohm-cm, which leads to its classification as a semiconductor. Since the transmission of electrical charge in moist concrete occurs through the movement of dissolved ions in the electrolytic solution, higher cement content and higher water content result in lower resistivity. High water content, however, is not acceptable for structural concrete, since it also results in lowered compressive strength and density. It has been found that there is a direct relationship between the degree of hydration of the cement paste and resistivity, yielding a linear relationship between resistivity and compressive strength of cement paste and concrete. That is, resistivity increases as the compressive strength increases.
Electrically conductive concrete may be produced by placing electrically conductive fibers and/or particles in close contact with each other so that a conductive network may be formed throughout the concrete. In conductive concrete, the transmission of electrical charge occurs mainly through the conductive additives, rather than through the electrolytic solution created in moist concrete. Such additives as carbon fibers, steel fibers, steel shavings, and carbon black have been found to be effective in modifying the conductivity of concrete into which they are blended. For example, U.S. Pat. No. 3,962,142 teaches the use of calcined oil coke and acetylene black aggregates in conductive concrete having satisfactory mechanical strength, while U.S. Pat. No. 5,908,584 teaches a mixture of graphite, amorphous carbon, and sand, comprising 25 to 75% of a cementitious composite useful for conducting floors, heating elements, and ground connectors.
Electrically conductive concrete and controlled low-strength materials would be advantageous where lowered electrical resistance may be sought, such as for use in structures where it is necessary to protect electrical equipment from lightning strikes. Accordingly, a means to reduce the electrical resistance of concrete or controlled low-strength materials, or to increase the conductivity thereof, is of interest in the building industry, for example. Further, since high carbon content fly ash is readily available as a waste product, and carbon is known to be highly conductive, its use as an additive to concrete or controlled low-strength materials to lower electrical resistance has now been investigated.
The present invention provides a means to increase the conductivity of concrete by the addition of high carbon content fly ash. The addition of such amounts of high carbon content fly ash has been found to lower the electrical resistance of the product concrete without seriously adversely affecting the mechanical properties thereof. The present invention also provides a means to increase the conductivity of controlled low-strength materials, by the addition of high carbon content fly ash. The addition of such amounts of high carbon content fly ash has been found to lower the electrical resistance of the product controlled low-strength material, without seriously adversely effecting the mechanical properties thereof. Moreover, this method provides a means for the environmentally acceptable beneficial utilization of such fly ash, as well as providing a low cost construction material having properties suitable for use in such applications as would benefit from an electrically conductive concrete or controlled low-strength material, such as in grounding structures for protection against lightning strikes or random electrical discharge.
Thus, it is an advantage of the present invention to provide an inexpensive method for providing electrically conductive construction materials.
It is a further advantage to provide an economically advantageous means of beneficial utilization of a waste by-product.
It is another advantage to provide electrically conductive concrete that can be inexpensively manufactured, and to provide a dry concrete mixture that may be packaged for sale to the public for individual use by consumers wishing to provide an electrically conductive concrete structural element.
It is thus another advantage to provide a dry mix for a conductive concrete, which can be inexpensively packaged, delivered, mixed and utilized as desired.
These and other features, aspects, and advantages of the present invention will become better understood upon consideration of the following detailed description, and appended claims.
It has been discovered that controlled low-strength materials and concrete can be made less electrically resistant by the addition of fly ash having a high carbon content. When using the term xe2x80x9ccarbon content in fly ashxe2x80x9d, the carbon content is as measured by loss on ignition. By the term xe2x80x9chigh carbon contentxe2x80x9d, it is meant that the carbon content of the fly ash is greater than or equal to about 12% by weight. The carbon present in such fly ash provides an electrical pathway throughout the composition for conduction of electricity, without a severe deleterious effect upon mechanical properties, such as compressive strength, of the concrete or controlled low-strength material, thus permitting its use in construction.
Electrical properties of concrete are of concern in some applications where inadequate resistivity of concrete affects electrical signaling systems, such as railway ties, or in structures in which concrete is used for protection from stray currents. Electrical properties of concrete are also of interest in such applications as cathodic protection of reinforced concrete; electrical grounding; deicing of airport runways, and bridge decks, by electrical heating; and non-destructive testing of concrete. Conductive concrete or controlled low-strength materials have, in the past, been made by placing electrically conductive material throughout the material. Electrically conductive additives have included carbon fibers, steel fibers, steel shavings, carbon black, coke breeze, etc.
As is well known in the art, conventional concrete is a dense hard hydrated mass produced from mixtures of portland cement, coarse aggregate, fine aggregate, and water. Concrete mixtures generally use only sufficient water to make the mixture placeable and to yield hardened concrete having a compressive strength of at least 13.8 MPa (2000 psi.) after 28 days. Portland cement is a well known cement that upon mixing with water binds or unites the other materials present in the mixture into concrete. The portland cement is typically a Type I, II, III, IV and V portland cement. The coarse aggregate conventionally comprises particles that are greater than about 0.375 inches (9.5 millimeters) in size and may be gravel, granite, limestone, shale, and the like. The fine aggregate employed in portland cement concretes is most often sand (silica) comprised of particles less than about 0.375 inches (9.5 millimeters) in size, typically equal to or less than about 0.1875 inches (4.76 millimeters) in size.
A concrete in accordance with the present invention is formed from a composition including portland cement, aggregate, water, and a high carbon content fly ash that provides an electrical pathway throughout the concrete for conduction of electricity, without a severe deleterious effect upon mechanical properties, such as compressive strength, of the concrete. One example embodiment of the invention is a composition capable of setting to produce a concrete according to the present invention wherein the composition includes from about 1% to about 30% by weight of portland cement, from about 1% to about 20% by weight of fly ash having a carbon content as measured by loss on ignition of greater than 12%, from about 40% to about 90% by weight of an aggregate, and water in a sufficient amount such that the composition sets to a concrete having a compressive strength of at least 13.8 MPa (2000 psi.), where all weight percentages are percent by weight of the total composition. In one version of the invention, the composition sets to a concrete having an electrical resistivity of not more than about 130,000 ohm-centimeters. In another version of the invention, the composition sets to a concrete having an electrical resistivity of not more than about 30,000 ohm-centimeters. In yet another version of the invention, the composition sets to a concrete having an electrical resistivity of not more than about 11,000 ohm-centimeters. In still another version of the invention, the composition sets to a concrete having an electrical resistivity of not more than about 1500 ohm-centimeters.
A further embodiment of the invention is a composition capable of setting to produce a concrete according to the present invention wherein the composition includes from about 1% to about 30% by weight of portland cement, from about 1% to about 20% by weight of fly ash having an average carbon content as measured by loss on ignition of greater than 12%, from about 40% to about 90% by weight of aggregate, and from about 1% to about 20% by weight of water. Yet another embodiment of the invention is a composition capable of setting to produce a concrete according to the present invention wherein the composition includes from about 5% to about 15% by weight of portland cement, from about 1% to about 12% by weight of fly ash having an average carbon content as measured by loss on ignition of greater than 12%, from about 65% to about 85% by weight of aggregate, and from about 5% to about 15% by weight of water. Still another embodiment of the invention is a composition capable of setting to produce a concrete according to the present invention wherein the composition includes from about 5% to about 15% by weight of portland cement, from about 1% to about 12% by weight of fly ash having an average carbon content as measured by loss on ignition of greater than 12%, from about 20% to about 40% by weight of a fine aggregate having a particle size of 9.5 millimeters (0.375 inches) or less, from about 35% to about 55% by weight of a coarse aggregate having a particle size greater than 9.5 millimeters (0.375 inches), and from about 5% to about 15% by weight of water.
A composition capable of setting to produce a concrete according to the present invention may also include from about 1% to about 10% by weight of steel fibers. In one version of the composition, the weight ratio of fly ash to portland cement in the composition is about 0.5:1 to about 1:1. The composition may also include additional additives known in the art including, without limitation, kiln dust, silica fume, superplasticizer, dispersing agent, or reinforcing fibers. However, the composition according to the invention may consist essentially of from about 5% to about 15% by weight of portland cement, from about 1% to about 12% by weight of fly ash having an average carbon content as measured by loss on ignition of greater than 12%, from about 20% to about 40% by weight of sand, from about 35% to about 55% by weight of crushed stone, and from about 5% to about 15% by weight of water.
A controlled low-strength material in accordance with the invention shares properties with both soils and concrete. The CLSM is prepared from materials (i.e., portland cement, water, optionally coarse aggregate, and optionally fine aggregate) similar to the concrete described above but also exhibits properties of soils. The specific properties of the CLSM depend on the ingredients in the mixture. The flowability of CLSM distinguishes it from other fill materials. Because of the flowability of the CLSM, the CLSM is self-leveling and self-compacting, i.e. the CLSM does not need conventional placing and compacting equipment. One version of a composition according to the invention for producing a CLSM in accordance with the present invention results in a CLSM having a compressive strength of 8.3 MPa (1200 psi) or less at the conventional 28 day testing period, while another version of a composition according to the invention produces a CLSM in accordance with the present invention that has unconfined compressive strengths of 2.1 MPa (300 psi) or less at the conventional 28 day testing period in order to allow future excavation. One controlled low-strength material in accordance with the present invention is formed from a composition including portland cement, water, and a high carbon content fly ash that provides an electrical pathway throughout the controlled low-strength material for conduction of electricity, without a severe deleterious effect upon mechanical properties, such as compressive strength, of the controlled low-strength material. Another controlled low-strength material in accordance with the present invention is formed from a composition including portland cement, aggregate, water, and a high carbon content fly ash.
One example embodiment of the invention is a self-compacting, cementitious flowable fill composition capable of setting to produce a controlled low-strength material according to the present invention wherein the composition includes from about 1% to about 20% by weight of portland cement, from about 18% to about 85% by weight of fly ash having a carbon content as measured by loss on ignition of greater than 12%, and water in a sufficient amount such that the composition sets to a material having a compressive strength of 8.3 MPa (1200 psi.) or less, where all weight percentages are percent by weight of the total composition. Another example embodiment of the invention is a self-compacting, cementitious flowable fill composition capable of setting to produce a controlled low-strength material according to the present invention wherein the composition includes from about 1% to about 20% by weight of portland cement, from about 18% to about 85% by weight of fly ash having a carbon content as measured by loss on ignition of greater than 12%, from about 40% to about 90% by weight of an aggregate, and water in a sufficient amount such that the composition sets to a material having a compressive strength of 8.3 MPa (1200 psi.) or less, where all weight percentages are percent by weight of the total composition.
A further embodiment of the invention is a self-compacting, cementitious flowable fill composition capable of setting to produce a controlled low-strength material according to the present invention wherein the composition includes from about 1% to about 10% by weight of portland cement, from about 40% to about 65% by weight of fly ash having an average carbon content as measured by loss on ignition of greater than 12%, and from about 30% to about 50% by weight of water. Still another embodiment of the invention is a self-compacting, cementitious flowable fill composition capable of setting to produce a controlled low-strength material according to the present invention wherein the composition includes from about 1% to about 10% by weight of portland cement, from about 40% to about 75% by weight of an aggregate, from about 18% to about 35% by weight of fly ash having an average carbon content as measured by loss on ignition of greater than 12%, and from about 5% to about 35% by weight of water. Preferably, the aggregate used in the CLSM composition is selected from concrete sand, crushed limestone, and mixtures thereof. The CLSM composition may also include additional additives known in the art; however, the CLSM composition according to the invention may also consist essentially of the components described above. Preferably, the weight ratio of fly ash to portland cement in the CLSM composition is greater than 10:1. In one version of the invention, the composition sets to a controlled low-strength material having an electrical resistivity of not more than about 6000 ohm-centimeters. In another version of the invention, the composition sets to a controlled low-strength material having an electrical resistivity of not more than about 3500 ohm-centimeters. In yet another version of the invention, the composition sets to a controlled low-strength material having an electrical resistivity of not more than about 100 ohm-centimeters.
A conductive concrete or controlled low-strength material produced in accordance with the present invention has many applications. For example, in one application, conductive concrete or controlled low-strength materials can be used as a substitute for materials which are used to absorb and ground various types of energy for military and other government applications. These uses may include absorption of direct energy, energy feedback and amplification, and EMP pulses. The conductive concrete or controlled low-strength materials would replace or supplement traditionally used grounding materials and be connected by grounding loops. Due to the engineering properties of the conductive concrete or controlled low-strength materials, the absorption should cover the entire energy range of the electromagnetic spectrum.
A conductive concrete or controlled low-strength material produced in accordance with the invention also would be beneficial when used in structures that may be struck by lightning. Lightning research photographs indicate that the initial lightning energy dissipates in 15 to 60 meter channels longitudinally across the surface of the earth. This is in contrast to the perception that all energy conducts into a finite earth point. The image appears similar to multiple lava flow channels emanating from a single volcano cone. Present buried grid systems not only do not prevent these phenomena, but may in fact enhance the effect. It suggests that earth""s inability to absorb large energy transfer at a finite point within the extremely short time event contributes to dangerous and damaging voltage scenarios. Extremely high voltage gradients result in physical structure and electronic equipment failures and indicate the need for an improved conduction transfer media to couple lightning energy to the earth. A conductive concrete or controlled low-strength material produced in accordance with the invention: (1) provides for low inductance, low resistance and subsequently low impedance values for all frequencies up to 1 Mhz; (2) conducts energy efficiently across and through its surface without damage while providing true equalized ground potential rise values; (3) conducts energy efficiently into the earth quickly and seamlessly by providing the lowest impedance-coupling path; (4) provides for long term, high strength physical properties; and (5) is compatible with copper, aluminum and galvanized steel products. Specific applications include: (1) microwave, radio, and cellular sites; (2) electrical power and substation plants; (3) electric distribution poles; and (4) high tech building systems.
Microwave, radio, and cellular site towers risk lightning strikes. Using a conductive concrete or controlled low-strength material produced in accordance with the invention in footings and large pads reduces ground potential rise by lowering the impedance between the lightning stroke and the earth. The event duration may also decrease due to shorter energy transfer times. A conductive concrete or controlled low-strength material produced in accordance with the invention could replace the typical wood, metal or standard concrete enclosure. Enhanced protection and reduced electronic equipment damage is achieved through the reduction of unacceptable voltage gradients. Using a conductive concrete or controlled low-strength material produced in accordance with the invention in the radial ground system will enhance energy transfer away from the site. The entire design package reduces risk to site failures thus improving revenue through reduced maintenance. A conductive concrete or controlled low-strength material produced in accordance with the invention will contribute to improved lightning protection efficiency at microwave, radio, TV, and cellular tower sites. Presently, the sites rely on copper wire grounding designs to lower earth resistivity to conduct lightning contacts to ground while providing an equalized ground potential rise within the protected site area. However, voltage gradients continue to exist resulting in equipment and revenue losses.
Electrical power and substation plants require above grade facilities grounded to large buried ground grid systems. This provides for efficient electrical operation of equipment and safe low step and touch ground potentials for operating personnel. Using a conductive concrete or controlled low-strength material produced in accordance with the invention to replace or enhance foundations and buried ground grid systems allows for integral above and below grade ground connections that are efficient, low maintenance, easy to test and resilient to damage. A conductive concrete or controlled low-strength material produced in accordance with the invention would provide high conductance paths for low and high frequency current transfers. Applications requiring low earth resistance values that are difficult to achieve are a particularly attractive solution to achieve satisfactory earth resistance values.
State regulations require electric distribution pole lines to be grounded periodically. While the primary purpose is system protection, stabilization, and elimination of stray voltage problems, surge protection devices connect to the same ground system. This results in a high impedance path for lightning and transient energy conduction. Surrounding the direct buried wood or metal pole base with a conductive concrete or controlled low-strength material produced in accordance with the invention increases fault and lightning protection by providing a low impedance conduction path to earth. In addition, increasing the large contact area between a conductive concrete or controlled low-strength material produced in accordance with the invention and earth enhances conductance, reduces wood pole rot at the earth line, and expedites installation.
The proliferation of xe2x80x9chigh techxe2x80x9d voice and data systems applications is requiring grounding performance evaluations in commercial facilities. Plastic utility service conduits to commercial and residential facilities remove traditional sources for attaching main ground systems. This increases the risk of communication equipment downtime and damage due to electrical system aberrations and lightning. National standards are directing building designer""s attention to providing efficient local ground systems such as external or incorporated grounding rings to replace inefficient ground systems. A conductive concrete or controlled low-strength material produced in accordance with the invention will complement designer""s options by allowing building steel components to couple directly to earth via a conductive structural medium. The transfer of lightning energy, stabilized equipotential ground system and presence of an efficient signal reference ground will contribute to high communication performance and reliability.
The invention is further illustrated in the following Examples which are presented for purposes of illustration and not of limitation.